Carbon resistive paste is typically screen printed on an insulating substrate and then baked, providing a resistive film. This resistive film is utilized as the resistor of, for example, fixed resistors and semi-fixed resistors.
Conventional carbon resistive paste is generally prepared by adding inorganic fillers, such as silica and talc; resins, such as an epoxy resin and a phenol resin; and solvents, such as ethanol, methanol, terpineol, diethylene glycol monoethyl ether and ethylene glycol monoethyl ether to an electrically conductive component, such as carbon or a mixture of carbon and graphite, and kneading the resulting mixture. In addition, organic fillers are sometimes added. Examples of these carbon resistive pastes are disclosed in U.S. Pat. Nos. 3,682,839 and 3,686,139.
Conventional resistive pastes, however, have the disadvantage of providing resistive films that are readily subject to serious deterioration. For example, when the resistive film is used as the resistor of a semi-fixed resistor and a slider is moved in contact with the resistive film, the change in resistance value after 100 rotations reaches .+-.10%. This is believed to be due to the fact that the resistive film is pressed when the slider is moved in contact therewith and, therefore, the distances between the electrically conductive component (e.g. carbon particles) is shortened, resulting in a decrease in contact resistance. In this case, the resistance value decreases. On the other hand, if the pressure of the slider is high, the resistive film is removed by mechanical abrasion. In this case, the resistance value increases.
Thus it is believed that serious changes in the resistance value of conventional resistive films are due to the softness of the baked film itself.